Field of the Invention
The invention relates to a configuration of a ferromagnetic, electrically conductive bearer or carrier body and winding elements, for instance being part of a heavy-duty electric machine with an electrical power consumption or output of more than approximately 20 MVA and preferably more than approximately 50 MVA. In particular, the invention relates to such a configuration for use as a stator in a turbogenerator.
A winding element in such a configuration typically has a basic unit with at least one electrically highly conductive metal wire or metal rod, and in particular a number of such metal rods, and is surrounded by an insulating sleeve being enveloped by an electrically semiconductive protective layer. The insulating sleeve of a winding element typically is formed of a material that contains mica and that is intended to be, or is, impregnated with a filler. Typically, it is a fine-mica-based tape which is wound up onto the basic unit, and the filler is a synthetic resin, preferably a hot-curing epoxy resin system, such as a hot-curing mixture of an epoxy resin and an acid anhydride. A metal wire or metal rod located in a winding element can be hollow, in order to carry a flowable coolant. Winding elements in the form of winding rods with rod-shaped basic units, and in particular with approximately straight basic units having bent ends, as well as in the form of shaped coils with basic units in the form of wound-on wires, are known. As a rule, shaped coils have two approximately straight segments, each to be placed in separate grooves. Winding rods are typically used in very heavy-duty dynamoelectric machines, for instance in turbogenerators, while shaped coils are preferably used in low-capacity dynamoelectric machines.
The most widely used method to produce a component with winding elements for a heavy-duty dynamoelectric machine requires the preparation of winding elements that are already provided with the impregnation of their insulating sleeves and that have received the requisite external shape by curing of the impregnation in special press molds, before their insertion into the component. So-called whole-part impregnation processes have also meanwhile come into use. In those processes, the winding elements receive their filler impregnations only after installation in the corresponding carrier body. The introduction of the whole-part impregnation process rendered the previously required precise shaping by pressing or the like largely superfluous. In the whole-part impregnation process, the grooves containing the winding segments in a carrier body are also substantially completely filled with the filler, so that the winding elements are maximally immovably fixed in the grooves of the carrier body, without additional wedging provisions of the kind that had always been required previously.
When a configuration that is to be impregnated and includes a carrier body and winding elements is constructed, the fact that the configuration is exposed to changing temperatures in a range from typical ambient temperatures to more than 100.degree. C., not only in the impregnation with filler and the ensuing curing of the filler, but in later operation in an electrical machine as well, must be taken into account. Due to the differing thermal expansion coefficients of the carrier body (which is typically a ferromagnetic metal) and the filler (which as a rule is epoxy resin), the possibility that thermal strains and fissuring may occur in the filler must be weighed and taken into account. It is already known to insert a separating layer between each winding element and at least one wall of the groove into which it is placed, with the strength of the separating layer being markedly reduced as compared with the strength of other layers in the groove, so that the fissuring occurs preferentially in the separating layer. In order to avoid corona discharges in such fissures (a danger that exists especially in the stators of heavy-duty electrical machines), the separating layer is electrically shielded by being inserted between two layers that are joined together and are at least weakly electrically conductive.
The electrically weakly conductive layer that is immediately adjacent the winding element can then be the protective layer that is typically already associated with the winding element to provide "external corona protection". The functions of the other electrically at least weakly conductive layer, which is opposite the protective layer behind the separating layer, can be assumed by the groove wall itself under some circumstances. However, an additional electrically conductive or weakly conductive intermediate layer may also be inserted.
Other embodiments of employing the whole-part impregnation process in the production of components for heavy-duty dynamoelectric machines can be found in Published International Application WO 91/01059, corresponding to U.S. Pat. No. 5,030,870, and in Published European Application No. 0 379 012A2. Those references both pertain to the production of configurations of ferromagnetic and electrically conductive carrier bodies and winding rods that are intended to be subjected to the whole-part impregnation process as described above. The second of those references moreover provides indications of a way in which the retention of the winding rods in a carrier body may be constructed, while exploiting the advantages that result from the whole-part impregnation process.
Other information on the whole-part impregnation process in the production of components for dynamoelectric machines may be found in both German Published, Non-Prosecuted Application DE 36 36 008A1 and U.S. Pat. No. 3,990,029. However, both of those references relate to configurations with winding bars having insulating sleeves which are already finally impregnated by the time they are installed in the carrier body.
Information on producing the winding elements and windings of electric machines and information on insulations in machines subject to heavy electrical and thermal loads may be found in the book entitled "Herstellung der Wicklungen elektrischer Maschinen" [Production of Windings for Electrical Machines], edited by H. Sequenz, Springer-Verlag, Vienna and New York 1973. A detailed discussion of mica, the material that is highly important for insulations in electrical machines, is found in the book entitled "Glimmer and Glimmererzeugnisse" [Mica and Mica Products], by H.-W. Rotter, published by Siemens AG, Berlin and Munich 1985. Particularly that latter work contains information on the use of laminated mica, which is understood to mean sheets produced by splitting large mica crystals, and fine mica, which is understood to be a paper-like product of finely ground mica. Laminated mica and fine mica are preferably used in the form of mica tapes, which are made from the respective mica product on a substrate such as Japan paper, glass cloth, or synthetic-fiber cloth. According to the second reference above, fine mica is the preferred product for making insulation elements in machines which are subject to heavy thermal and electrical loads, since a laminated mica insulation has a substantially greater tendency than fine mica to form voids and could thus possibly cause corona discharges in insulations, because of the resilience and the tendency toward further splitting that the sheets of laminated mica have.
A configuration of a ferromagnetic and electrically conductive carrier body and winding rods to be impregnated with a filler by the whole-part impregnation process, and in which a separating layer that is electrically shielded to prevent corona discharges is disposed between each winding rod and at least one wall of the associated groove as already explained, is shown in Published International Application WO 91/01059, corresponding to U.S. Pat. No. 5,030,870. The separating layer is formed with the aid of substances that cannot be wetted by the filler. Such substances may be coatings or impregnations of textiles or foils, which may optionally be permeable to the filler by means of perforations or the like. In order to assure the shielding of the separating layer, that layer is located between two electrically weakly conductive layers. Those latter layers may either touch one another through the separating layer and thus form electrical contacts, or a metal contact conductor may be used that is in contact with both weakly conductive layers.